The conversion of chromium oxide to chromium carbide

The thermodynamic equilibria between chromium oxide, chromium carbide, and a CO-CO₂ mixture was calculated for various conditions. Experimental results on the rate of the Cr₂O₃ conversion to chromium carbide in CO-CO₂ mixtures at 1200°C are reported. The conversion rate was low and was strongly dependent on the carbon activity.     

SiC颗粒表面Mo涂层的制备与分析

采用磁控溅射法在碳化硅(SiC)颗粒表面成功制备了金属钼(Mo)涂层,分析了Mo涂层的成分和形貌;为改善初始涂层成分和形貌,对镀Mo改性SiC复合粉体进行了不同工艺的结晶化热处理,重点研究了热处理对SiC颗粒表面Mo涂层形貌和成分的影响。结果表明,磁控溅射法能够在SiC颗粒表面沉积Mo涂层,随磁控溅射时间的延长,SiC颗粒表面Mo涂层的粗糙度增大,但磁控溅射后SiC颗粒表面Mo涂层为非晶态。热处理能够有效改善SiC颗粒表面Mo涂层的成分、形貌及结晶状态,在600~1200℃之间结晶化热处理过程中,随热处理温度升高,SiC颗粒表面Mo涂层形貌主要经历了以下4个阶段变化:Mo涂层初步致密化—Mo的结晶致密化—Mo涂层的聚集长大—Mo与SiC之间化学反应;相应的Mo原子的存在状态也经历了如下变化:非晶态Mo原子—晶态Mo原子—Mo_2C和MoSi_2。其中800~900℃之间为最佳热处理温度,此时Mo涂层致密均匀包覆完整。SiC表面连续均匀致密的Mo涂层,有利于改善SiC颗粒增强金属基复合材料中基体与增强体之间的界面结合并控制不利界面反应,有利于复合材料综合性能的提高,必将扩大SiC颗粒作为增强体的应用范围。     

Lanthanide salts as thermal conversion coatings on silicon carbide reinforced AA5A06 aluminium metal matrix composite

In this study, cerium and lanthanum chloride binary mixtures conversion coatings were investigated as corrosion inhibitors for silicon carbide particulates reinforced AA5A06 aluminium composite and AA5A06 aluminium alloy. Electrochemical impedance spectroscopy and potentiodynamic polarization tests revealed that cerium (III) chloride additions markedly improved the pitting corrosion resistance as compared to lanthanum (III) chloride additions, with maximum increase noted for samples treated with 750 ppm cerium (III) + 250 ppm lanthanum (III) chlorides. Scanning electron microscopy, energy dispersion spectroscopy and X‐ray photoelectron spectroscopy examinations revealed the existence of a heterogeneous layer formed by precipitation of cerium/lanthanum oxide/hydroxide on the cathodic intermetallics and an aluminium oxide film on the rest of the metal matrix. Improved corrosion resistance was attained for SiC particulates reinforced aluminium composite by means of the cerium/lanthanum conversion treatment, due to the presence of mainly cerium (IV) species as a result of oxidation of cerium (III), which provides auto‐protection properties.     

Electrochemical corrosion of silicon carbide ceramics

Recently, SiC‐based ceramics have been found to exhibit corrosion damage patterns, which can only be explained by electrochemical processes. Therefore, the current work focusses on the electrochemical test procedures to determine the corrosion behaviour of solid state sintered silicon carbide (SSiC) ceramics in acidic and alkaline media. The corrosion current densities have been determined from linear voltammetric scans. At anodic polarization potentials, electrochemically induced etching patterns were observed in alkaline solution. The formation of pores and crevices during electrochemical oxidation in acidic solution could be monitored by field emission scanning electron microscopy (FESEM) in addition to transmission electron microscopy (TEM). Impedance spectra measured after anodic polarization could be described by the assumption of a pore model equivalent circuit.     

Simultaneous chromizing — Aluminizing coating of low-alloy steels by a halide-activated,pack-cementation process

The simultaneous chromizing — aluminizing of low-alloy steels has achieved Kanthal-like surface compositions of 16–21Cr and 5–8 wt.% Al by the use of cementation packs with a Cr-Al masteralloy and an NH₄Cl activator salt. An initial preferential deposition of Al into the alloy induces the phase transformation from austenite to ferrite at the 1150°C process temperature. The low solubility of carbon in ferrite results in the rejection of solute C into the austenitic core, thereby preventing the formation of an external Cr-carbide layer, which would otherwise block aluminizing and chromizing. The deposition and rapid diffusion of Cr and Al into the external bcc ferrite layer follows. Parabolic, cyclic-oxidation kinetics for alumina growth on the coated steels in air were observed over a wide range of relatively low temperatures (637–923°C).     

碳化硅材料在冰晶石熔液中的侵蚀行为

研究了气、冰晶石、铝液三相区介质在温度为900℃时对碳化硅材料的侵蚀行为。分别用光学显微镜和扫描电镜观察了试样在熔液区域的横截面以及试样与三相区的界面形貌。研究结果表明：碳化硅材料在熔融的冰晶石混合介质中表现出良好的抗腐蚀性能；铝液对碳化硅材料有侵蚀作用，而电解质、空气界面对碳化硅有较强的腐蚀行为。分析和探讨了各相介质对碳化硅材料的腐蚀机理以及腐蚀产物的相组成。     

拓扑法计算硅/碳化硅材料的电阻率

用拓扑关系将硅 /碳化硅材料转化为具有 3个不同显微结构单元的整体 ,建立了等效电路 ,结合相关试验 ,计算了不同显微组织的硅 /碳化硅材料的电阻率。得到的理论计算结果与实验值的综合相对误差仅为3.9% ,分析比较了各相含量和分布形态对硅 /碳化硅材料整体电阻率的影响。结果表明 ,提高低电阻率硅的体积含量和其分布连续性 ,可降低材料的整体电阻率。该方法可用来预测要得到所需电阻率材料中应具有的硅含量和分布形态 ,为确定制备材料的显微结构设计提供指导。     

High temperature oxidation of silicon carbide based materials

The process of high temperature oxidation of two silicon carbide based materials differing by methods of their production and properties has been studied up to 1500°C in air. The oxidation was performed under the isothermal conditions and at the programmed heat rate of 10° per minute. It was found that the oxidation resistance of the material was the function of the presence of extrinsic metals having close affinity for oxygen. It was also found that under heating up to 1500°C in air phase transitions occurred in the SiC surface layer.     

Simultaneous chromizing-aluminizing coating of austenitic stainless steels

Chromium and aluminum were simultaneously co-deposited by diffusion into austenitic stainless steel substrates, by a single-step, pack-cementation process. The mechanism for the formation of diffusion-coated products on 304 and 316 stainless steels and on Incoloy 800 is discussed. The morphologies of the phases formed at the surface, i.e., an external beta layer and an underlying multiphase interdiffusion zone, are presented. The formation of the brittle, , outer layer was minimized by variations in the pack composition and activator. The coated 304 and 316 steels exhibited excellent scaling resistance upon oxidation in air at 1000°C.     

Study on plasma sprayed boron carbide coating

The microstructure, phase composition, and mechanical properties of boron carbide coatings formed by atmospheric plasma spraying (APS) are studied in the present work. The boron carbide coating with high microhardness and low porosity could be produced by APS. The decomposition of boron carbide powder during the plasma spray process would result in the formation of the B_xC phase and an increase of the carbon phase, which is confirmed by transmission electron microscopy, x-ray photoelectron spectroscopy, and x-ray diffraction results.     

Formation and Characterization of Titanium Modified Aluminide Coatings on IN738LC

Up to now, the aluminide coatings used to protect industrial components at high temperature and corrosive environments have been modified by Pt, Cr, Si and Ni. In this investigation, aluminide coatings were modified by titanium and the microstructural feature and formation mechanism were evaluated. The coatings were formed on a Ni-based superalloy(IN738LC) by a two stage process including titanizing at first and aluminizing thereafter. Pack cementation titanizing performed at temperatures 950℃ and 1050℃ in several mixtures of Ti, Al2O3 and NH4Cl. At the second stage,aluminum diffused into surface of the specimens by an industrial aluminizing process known as Elcoatl01(4 hrs at 1050℃C). The modified coatings were characterized by means of standard optical microscopy, scanning electron microscopy,energy dispersive spectroscopy and X-Ray diffraction methods. The results show that Ti in the coatings is mainly present in the form of TiNi and Al67CrsTi25. Titanium modified coatings grew with a mechanism similar to simple aluminizing; this includes inward diffusion of Al from the pack to the substrate and then outward diffusion of Ni from the substrate to the coating. The advantages and characteristics of this two-stage modified coating is discussed and the process parameters are proposed to obtain a coating of optimum microstructure.     

Formation and Characterization of Titanium Modif Aluminide Coatings on IN738LC

HISTORICALLY superalloys have been used in gasturbine applications with a protective coating to combatoxidation problems in aero engines and hot corrosionproblems in industrial and marine engines.Theprotective coatings used are predominantly of thediffusion type produced by variety of techniques suchas pack cementation,chemical vapor deposition,slurryfusion and hot dipping[1,2].The purpose of this typeof coatings is to increase the amount of Al at thesurface of a component,so that a protec…     

Formation and Microstructural Characterization of Silicon-modified Aluminide Coating on Nickel-based Superalloy IN738LC

ALUMINIDE COATINGS are used extensively forprotecting gas turbine hot section components due totheir high oxidation resistance and low capital cost.However,for some application,their properties must bemodified by adding elements such as Cr,Si and Pt tomeet high performance gas turbines requirements[l,2].Although silicon is less efficient as a protective scaleformer than chromium and aluminum,silicon-modifiedaluminide coatings,have shown better hot corrosionresistance(specially to type …     

微波烧结碳化硅的制备

以硅粉为硅源,乙炔炭黑为碳源,通过微波加热的方法制备碳化硅。研究反应物密实度和反应物粒度对产物碳化硅粒度和形貌的影响,利用SEM和TEM等手段观察碳化硅的形貌。结果表明:采用自由堆积的乙炔炭黑和硅粉为反应物时,硅粉、碳粉表面存在的氧与其发生反应生成的Si O和CO,再通过气-气反应生成Si C晶须,生成产物包括Si C颗粒和晶须。当反应物加压后,生成的Si O与Si和C反应生成Si C颗粒,产物中只包括Si C颗粒。乙炔炭黑和硅粉反应过程中Si C通过扩散机制生成,碳颗粒的粒度决定了生成的Si C的粒度。     

Codeposited chromium and silicon diffusion coatings for Fe-Base alloys via pack cementation

The simultaneous deposition of Cr and Si into plain carbon, low-alloy, and austenitic steels using a halide-activated pack-cementation process is described. Equilibrium partial pressures of gaseous species have been calculated using the STEPSOL computer program to aid in designing specific processes for codepositing the desired ratios of Cr and Si into a given alloy. The calculations indicate that NaCl-activated packs are chromizing, while NaF-activated packs deposit more Si with less Cr. The use of a dual activator (e.g., NaF+NaCl) allows for the deposition of both Cr and Si in the desired amounts. Single-phase ferritic coatings (150–250 microns thick) with a surface concentration of 20–35 wt.% Cr and 2–4% Si have been grown on AISI 1018, Fe-2.25 Cr-1.0Mo-0.15C, and Fe-0.5 Cr-0.5 Mo-0.2C steels using packs containing a 90 wt.% Cr-10Si binary source alloy, a NaF+NaCl activator, and a silica filler. Two-phase coatings (approximately 75 microns thick) containing 20–25 wt.% Cr and 2.0–2.4% Si have been obtained on 304 stainless steel using packs containing a 90 wt.% Cr-10Si binary source alloy, a NaF activator, and an alumina filler. The same pack chemistry allowed the diffusion of Cr and Si into the austenitic Incoloy 800 alloy without a phase change. A coated Fe-2.25 Cr-1.0 Mo-0.15 C coupon with a surface concentration of Fe-34 wt.% Cr-3Si was cyclically oxidized in air at 700°C for over four months and 47 cycles. The weight gain was very low (<0.2 mg/cm²) with no scale spalling detected. Coated coupons of AISI 1018 steel, and Fe-0.5 Cr-0.5 Mo-0.2C steel have shown excellent oxidation-sulfidation resistance in reducing, sulfur-containing atmospheres at temperatures from 400 to 700°C and in erosion and erosion-oxidation testing in air at 650 and 850°C.     

Effects of light-mass structural forms on silicon carbide mirror

To reduce the mass of the mirror, silicon carbide was used as the material and four light-mass structures were designed. The properties of SiC mirror open back structure with triangular cell, open back structure with hexagonal cell, sandwich structure with triangular cell, sandwich structure with hexagonal cell, were analyzed by using finite element method. The results of the static, dynamic, and thermal properties of the four kinds of SiC mirror indicate that the surface figures of the SiC mirrors are all satisfactory with the design requirements. The properties of the mirrors with sandwich structure are better than those with open back structure, except the high cost. And the mirror with triangular cell has better combination properties than the mirror with hexagonal cell. Considering the overall performance and the cost, open back structure with triangular cell is the most suitable for the SiC mirror.     

Preparation and properties of continuous Al-containing silicon carbide fibers

Continuous SiC(OA1) fibers, named KD-A fibers, were prepared by the melt-spinning of ceramic precursor polyaluminocarbosilane, air-curing, and pyrolizing at 1 300℃. These fibers contained small amount of aluminum and 7%-9% oxygen. The KD-A fibers were converted into sintered SiC(A1) fibers, named KD-SA, by sintering at 1 800℃. The fibers were characterized by chemical analysis, tensile strength test, SEM and XRD. The tensile strength, elastic modulus and diameter of the KD-A fibers are 2.6 GPa, 210 GPa, 12- 14μm, respectively. The KD-A fibers have higher thermal stability, more excellent oxidation resistance than the Nicalon fibers. The properties of the KD-A fibers have reached the level of Hi-Nicalon fibers. The tensile strength, elastic modulus and diameter of the KD-A fibers are 2.1 GPa, 405 GPa, 10 - 12μm, respectively. The KD-SA fibers with nearly stoichiometric component have stable performance at high temperature, and better creep resistance than the Tyranno SA fibers.     

Morphology, structure and formation mechanism of silicide coating by pack cementation process

The MoSi2 coating on C 103 niobium based alloy was prepared by pack cementation method. The formative mechanism, morphology and structure of coating were investigated. The silicide coating was formed by reactive diffusion obeying parabolic rule during pack cementation process. It is found that the composite structural coating is composed of three inferior layers as follows. The main layer is composed of MoSi2, the two phases＇ transitional layer consists of NbSi2 and a few NbsSi3 and the diffuse layer is composed of NbsSi3. The dense amorphous glass layer formed on the surface at high temperature oxidation circumstance can effectively prevent the diffusion of oxygen into coating.     

Dielectric properties of doped silicon carbide powder by thermal diffusion

The doped SiC powders were prepared by the thermal diffusion process in nitrogen atmosphere at 2 000℃. Graphite film with holes was used as the protective mask, The dielectric properties of the prepared SiC powders at high frequencies were investigated. The complex permittivity of the undoped and doped SiC powders was measured within the microwave frequency range from 8.2 to 12.4 GHz. The XRD patterns show that before and after heat treatment no new phase appears in the samples of undoped and nitrogen-doped, however, in the aluminum-doped sample the AIN phase appears. At the same time the Raman spectra indicate that after doping the aluminum and nitrogen atoms affect the bond of silicon and carbon. The dielectric real part （ε） and imaginary part （ε＂） of the nitrogen-doped sample are higher than those of the other samples. The reason is that in the nitrogen-doped the N atom substitutes the C position of SiC crystal and induces more carriers and in the nitrogen and aluminum-doped the concentration of carriers and the effect of dielectric relaxation will decrease because of the aluminum and nitrogen contrary dopants.     

Cerium‐based thermal conversion treatments on silicon carbide reinforced 2009 aluminum alloy composites

The conversion coatings on SiC_p/2009 aluminum (Al) composites and 2009 Al alloy were obtained by immersing the samples into the cerium (Ce)‐based solutions with varying the chemical concentration (CeCl₃·7H₂O, NaCl), solution pH/temperature, immersing time, and drying temperature. The corrosion inhibition mechanism and kinetics were studied by using the electrochemical techniques and surface analyses. Potentiodynamic polarization tests showed that coatings improved corrosion resistance as compared to untreated samples and environmental factors played important role in the formation of conversion layer. Scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), and X‐ray photoelectron spectroscopy (XPS) analysis revealed that relatively high inhibition efficiency for composite with 25 vol% SiC_p was due to the existence of rather high surface area of cathodic intermetallics/SiC_p, which support to deposit Ce oxides/hydroxides on the entire surface.